Steel rail for high speed and quasi-high speed railways and method of manufacturing the same

ABSTRACT

The present discloses a steel rail for high speed and quasi-high speed railways and a manufacturing method thereof. The steel rail having a superior rolling contact fatigue property can be obtained by reducing content of carbon in conjunction with controlled cooling after rolling. The steel rail includes 0.40-0.64% by weight of C, 0.10-1.00% by weight of Si, 0.30-1.50% by weight of Mn, less than or equal to 0.025% by weight of P, less than or equal to 0.025% by weight of S, less than or equal to 0.005% by weight of Al, more than 0 and less than or equal to 0.05% by weight of a rare earth element, more than 0 and less than or equal to 0.20% by weight of at least one of V, Cr, and Ti, and a remainder of Fe and inevitable impurities. The steel rail manufactured according to the method of the present invention maintains the strength and hardness of the existing steel rail for the high speed railways, while enhancing the toughness, plasticity and yield strength, and an energy value required for initiating and expanding microcracks formed at the surface of the steel rail due to fatigue is increased, and thus under the same conditions, the rolling contact fatigue property of the steel rail can be improved, thereby finally improving the service lifetime and the transportation safety of the steel rail.

FIELD OF THE INVENTION

The present invention relates to a steel rail material, moreparticularly, a steel rail adapted to be used in a high speed orquasi-high speed railway and a method of manufacturing the same.

DESCRIPTION OF RELATED ART

There have been three main kinds of railways nowadays in the world,i.e., heavy haul railway, high speed railway, and mixed passenger andfreight railway. As for steel rails for the heavy haul railway, becauseof generally 25 t-40 t of an axle load of a train, great contact stressbetween wheel and rail, and harsh forces, a carbon steel or alloy steelrail having more than 0.75% of C, a tensile strength of 1200 MPa ormore, and a full pearlite structure is generally used to ensure that thesteel rail has excellent resistance to wear. As for the high speedrailway, since it is mainly used in passenger transport and the trainhas a light axle load, steel rails for the high speed railway aregenerally required to have an excellent antifatigue property. As for themixed passenger and freight railway, since it is used not only forpassenger transport, but also to ensure the particularity of cargotransport, the used steel rail is required to have both predeterminedresistance to wear and predetermined antifatigue property to reach abalance therebetween. As for the steel rails used for the mixedpassenger and freight railway, a hot-rolled or heat-treated steel railhaving 0.70%-0.80% of C and a tensile strength of 900-1100 MPa isgenerally used, a steel rail having a tensile strength of 1200 MPa maybe used for a railway having curve with a small radius, and the steelrails used for the mixed passenger and freight railway have ametallurgical structure with a dominant component of pearlite and,partially, a tiny amount of ferrite. Since the steel rails used for bothhigh speed and quasi-high speed railways are required to havepredetermined antifatigue properties, hot-rolled U71Mn steel railshaving a tensile strength of 900 MPa and 0.65%-0.76% of C are widelyused in the high speed and quasi-high speed railways.

However, practical application shows that a crack which has already beengenerated in an upper or side surface of a head portion of a steel railis difficult to be worn away due to a relatively light axle load ofgenerally 11-14 tons of a high speed train and little wear-out betweenwheel and rail in the practical operation, and under repeated wheel-railcontact force, propagation of the crack may be in turn aggravated,resulting in tendency of fracture of the steel rail, which seriouslyendangers running safety of the train. On the other hand, if a wear rateof the steel rail is improved by a method of only decreasing strengthand hardness of the steel rail, a plastic flow may occur in a surface ofthe steel rail to cause deviation in cross-sectional dimension of thesteel rail so that the train cannot run along the railway, and a servicelifetime of the railway may be also shortened due to excessive wear-outof the steel rail. Accordingly, as for the high speed or quasi-highspeed railways, a balance is difficult to be made between wear-out androlling contact fatigue of the hot-rolled steel rail having a dominantcomponent of pearlite.

In order to improve the rolling contact fatigue property of the steelrails for the high speed and quasi-high speed railways, there have beenmainly two methods at present. A first method is to periodically grindan upper end of the steel rail by using a railway-grinding train, butthis method has a problem in that the railway-grinding train isexpensive, and meanwhile, there is a high traffic density on the highspeed and quasi-high speed railways so that no sufficient grinding timecan be spared. A second method is to improve the wear rate of the steelrail surface so that a fatigue layer is worn away through continuouswheel-rail wear-out before fatigue damage occurs. The wearingcharacteristic of the steel rail is affected by its hardness, and thusthe hardness of the steel rail may be reduced so as to facilitatewear-out. However, simply reducing hardness may result in plasticdeformation occurring on an upper surface of the steel rail afterrunning a period of time, frequently accompanied by damages such ascrack and peeling, which also negatively effect the lifetime andtransportation safety of the steel rail.

In recent years, in order to improve contact fatigue damage property ofa steel rail for a high speed railway, a steel rail having a dominantcomponent of bainite, a small amount of martensite, and residualaustenite has been developed. Chinese Patent No. CN1074058C discloses abainite-based steel rail with excellent bonding characteristic in itswelding portion and a method of manufacturing the same. Thebainite-based steel rail includes 0.15%-0.40% of C, 0.1%-0.2% of Si,0.15%-1.10% of Mn, less than or equal to 0.035% of P and S, as well asCr, Nb, Mo, V, Ni and other elements.

However, in theory, a steel rail having a bainite structure, especiallya lower bainite structure, has a significantly improved toughness andplasticity and an advantage in running safety as compared with apearlite-based steel rail having the same strength level, but in termsof wear-out and rolling contact fatigue properties, its theoreticalvalues are not consistent with its practical values. The structure andperformance of bainite depend on morphologies, distribution andinteraction of ferrite and carbide. For example, the carbide issolid-solved in the ferrite or distributed along grain boundaries of theferrite, the steel rail may have significantly different hardness. Thehardness directly determines the wear property, and thus extremelystrict requirements for process control and production processes ofsteel rails are needed in order to obtain an ideal structural form. Inaddition, in the case of the bainite-based steel rail disclosed inChinese Patent No. CN1074058C, in order to obtain an ideal bainitestructure, a strict control process for the steel rail is required, alarge amount of valuable elements need to be added, causing themanufacturing cost of the steel rail to be much higher than the existingpearlite-based series rail, and even if the performances of the steelrail manufactured are excellent, it will be difficult to bemass-manufactured and widely used.

Therefore, manufacture of the bainite steel rail and wide applicationthereof to the high speed or quasi-high speed railway are limited due tostrict manufacturing process as well as addition of a quantity ofvaluable alloys, thus a high manufacturing cost equal to or more thantwice of the existing pearlite steel rails. In addition, it still needsto be further verified whether the fatigue property of the bainite steelrail is superior to that of the existing pearlite steel rail or not.

Thus, there is an urgent need for a pearlite-based steel rail which hasa low manufacturing cost while having excellent resistance to wear andfatigue damage to be suitable for high speed or quasi-high speed railwayapplications.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above describedproblems existing in the prior art, and to provide a steel rail suitablefor a high speed or quasi-high speed railway having an excellent rollingcontact fatigue property.

The present invention provides a steel rail for high speed andquasi-high speed railways including 0.40-0.64% by weight of C,0.10-1.00% by weight of Si, 0.30-1.50% by weight of Mn, less than orequal to 0.025% by weight of P, less than or equal to 0.025% by weightof S, less than or equal to 0.005% by weight of Al, more than 0 and lessthan or equal to 0.05% by weight of a rare earth element, more than 0and less than or equal to 0.20% by weight of at least one of V, Cr, andTi, and a remainder of Fe and inevitable impurities, wherein a headportion of the steel rail has a uniformly mixed microstructure ofpearlite and 15-50% of ferrite at a room temperature.

According to one embodiment of the present invention, the steel railincludes 0.45-0.60% by weight of C, 0.15-0.50% by weight of Si,0.50-1.20% by weight of Mn, less than or equal to 0.025% by weight of P,less than or equal to 0.025% by weight of S, less than or equal to0.005% by weight of Al, more than 0 and less than or equal to 0.05% byweight of a rare earth element, more than 0 and less than or equal to0.20% by weight of at least one of V, Cr, and Ti, and a remainder of Feand inevitable impurities. According to another embodiment of thepresent invention, the steel rail may include at least one of 0.01-0.15%of V, 0.02-0.20% of Cr, and 0.01-0.05% of Ti. According to yet anotherembodiment of the present invention, the steel rail may include at leastone of 0.02-0.08% of V, 0.10-0.15% of Cr, and 0.01-0.05% of Ti.

According to one embodiment of the present invention, the head portionof the steel rail has a uniformly mixed microstructure of pearlite and15-30% of ferrite at the room temperature.

The present invention provides a method of manufacturing the steel raildescribed above including smelting and casting molten steel, rollingsteel rail, controlled cooling after rolling, and air-cooling, whereinthe controlled cooling after rolling may include making the steel railstand upright on a roll table, transferring the steel rail to a heattreatment unit through rotation of the roll table, and blowing coolingmedium onto the steel rail by the heat treatment unit to uniformly coolthe head portion of the steel rail at a cooling rate of 1-4° C./s untila temperature of a top side of the head portion decreases to 350-550° C.

According to the present invention, the method may further include afterfinishing rolling during the rolling steel rail, cooling the steel railto a temperature lower than an austenitic phase zone, and then heatingthe steel rail to a temperature in the austenitic phase zone at a rateof 1-20° C./s, followed by the controlled cooling after rolling.

According to one embodiment of the present invention, the cooling mediummay be at least one of compressed air, a mixture of water and air, and amixture of oil and air.

According to the present invention, the smelting and casting moltensteel may include smelting the molten steel by using a converter, anelectric furnace or an open-hearth furnace, performing a vacuumtreatment on the molten steel, casting the molten steel to a billet or aslab, and cooling the billet or the slab or directly transferring thebillet or the slab to a heating furnace to increase a temperaturethereof. The rolling steel rail may include feeding a billet or acontinuously cast slab which has been heated to a certain temperatureand kept for a certain period of time into a rolling machine to roll thebillet or the continuously cast slab to a steel rail having a requiredcross-section. During the rolling steel rail, the temperature of thebillet or the continuously cast slab may be increased to 1200-1300° C.,and kept for 0.5-2 h.

According to the present invention, the method may further include afterthe controlled cooling after rolling, placing the cooled steel rail inthe air to be naturally cooled to a room temperature.

In the present invention, by reducing the content of carbon element in asteel rail, with controlled cooling after rolling, toughness andplasticity and a yield strength of the steel rail can be improved whilemaintaining the levels of strength and hardness of the existing steelrail for the high speed railway, and an energy value required forinitiating and expanding microcracks formed at the surface of the steelrail due to fatigue can be increased, and thus under the sameconditions, the rolling contact fatigue property of the steel rail canbe improved, thereby finally improving the service lifetime and thetransportation safety of the steel rail.

DESCRIPTION OF FIGURES

The above and other objects and feature of the present invention willbecome more apparent by the following description in conjunction withthe accompanying drawings, in which:

FIG. 1 is a schematic view illustrating wearing of a steel railaccording to the present invention and a steel rail according to theprior art;

FIG. 2 is a metallograph of a rail head structure of a steel railaccording to one embodiment of the present invention; and

FIG. 3 is a metallograph of a rail head structure of a steel railaccording to a comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With the development of high speed and quasi-high speed railways, steelrails are required to have excellent comprehensive performances toensure safety and longevity of high speed railways. A train runs alongsteel rails at a high speed, thus the steel rails are required to haveexcellent toughness and plasticity, and excellent rolling contactfatigue performance, in addition to an appearance with high flatness,high accuracy of geometric dimensions and defect-free. As for thecurrent steel rails used for high speed and quasi-high speed railways,wear-outs of surfaces the steel rails due to wheel-rail contact frictionneed to be decreased as possible so as to ensure a long lifetime;meanwhile, in order to ensure that microcracks which have been generatedin a surface of a steel rail can be timely worn away before expandinginwardly, a certain wear rate needs to be further ensured, which is incontradiction with increasing a service lifetime of the steel rail.However, both decreasing wear-out and improving rolling contact fatigueproperty seem to be not fundamentally resolved.

Therefore, in the present invention, by reducing the content of Celement in a steel rail, with controlled cooling after rolling,toughness and plasticity and a yield strength of the steel rail can beimproved while maintaining the levels of strength and hardness of theexisting steel rail for the high speed railway, and an energy valuerequired for initiating and expanding microcracks formed at the surfaceof the steel rail due to fatigue can be increased, and thus under thesame conditions, the rolling contact fatigue property of the steel railcan be improved, thereby finally improving the service lifetime and thetransportation safety of the steel rail.

In particular, the present invention provides a steel rail for highspeed and quasi-high speed railways including 0.40-0.64% by weight of C,0.10-1.00% by weight of Si, 0.30-1.50% by weight of Mn, less than orequal to 0.025% by weight of P, less than or equal to 0.025% by weightof S, less than or equal to 0.005% by weight of Al, more than 0 and lessthan or equal to 0.05% by weight of a rare earth element (RE), more than0 and less than or equal to 0.20% by weight of at least one of V, Cr,and Ti, and a remainder of Fe and inevitable impurities. Preferably, thesteel rail for high speed and quasi-high speed railways according to thepresent invention includes 0.45-0.60% by weight of C, 0.15-0.50% byweight of Si, 0.50-1.20% by weight of Mn, less than or equal to 0.025%by weight of P, less than or equal to 0.025% by weight of S, less thanor equal to 0.005% by weight of Al, more than 0 and less than or equalto 0.05% by weight of a rare earth element, more than 0 and less than orequal to 0.20% by weight of at least one of V, Cr, and Ti, and aremainder of Fe and inevitable impurities. In the following description,the contents of the mentioned substances are based on weightpercentages, unless stated otherwise.

The steel rail for high speed and quasi-high speed railways according tothe present invention has a uniformly mixed metallurgical structure ofpearlite and 15% to 50% of ferrite (preferably, pearlite and 15% to 30%of ferrite) at room temperature, an elongation after fracture of morethan or equal to 15%, a yield strength (R_(El)) of more than or equal to550 MPa, and a fracture toughness K_(IC) of more than or equal to 40MPam^(1/2) at −20° C.

Hereinafter, the reasons for limiting the chemical components of thesteel rail according to the present invention to the above-describedranges will be first described.

C is one of the most important and economical elements in the steel railto endow it with an appropriate strength, hardness and resistance towear. In the steel rail according to the present invention, when thecontent of C is less than 0.40% by weight, the wear property may bereduced because the amount of carbides in the metallurgical structure istoo small to be concentrated below a head tread of the steel rail,resulting in reduced service lifetime of the steel rail due to beingworn too fast; at the same time, due to reduction in hardness, a plasticflow zone is formed in the tread of the steel rail, and such defects asflash and the like are prone to be generated, endangering running safetyof a high speed train. In the steel rail according to the presentinvention, when the content of C is more than 0.64 wt %, the strengthand hardness of the steel rail will be excessively high by a subsequentheat treatment process. As for this, on the one hand, the cracks whichhave been generated can not be worn timely to expand, so that there isan increased tendency for the steel rail to be laterally fractured; onthe other hand, the excessively high hardness of the steel railaccelerates the wear rate of a wheel, significantly reducing the servicelifetime of the train. In addition, under the same conditions, theimprovement in the strength of the steel rail is necessarily accompaniedby reduced toughness and plasticity, which can not meet safetyrequirements as well. Therefore, the content of C is defined to bebetween 0.40% and 0.64% in the present invention so that a rigidityrequired for the steel rail can be better satisfied, while matching thehardness of the rail and the hardness of the wheel with each other andimproving safety of the rail in use. Preferably, the content of C isdefined to be between 0.45% and 0.60%.

Si, as a main added element in the steel rail, usually exists in ferriteand austenite in a form of solid solution to increase the strength ofthe metallurgical structure. In the steel rail according to the presentinvention, when the content of Si in the steel rail is less than 0.10%by weight, the amount of the solid solution will be too low, resultingin an unobvious strengthening effect, and when the content of Si is morethan 1.00% by weight, the toughness and plasticity, and ductility of thesteel rail will be reduced. In addition, when the content of Si in thesteel rail is relatively high, a lateral performance of the steel railmay be significantly deteriorated, negatively affecting the safety ofthe steel rail in use. Therefore, in the present invention the contentof Si is defined to be between 0.10% and 1.00%, especially when 0.15 wt%<Si %<0.50 wt %, the effect is remarkable.

Mn may form a solid solution together with Fe to improve the strength offerrite and austenite. Meanwhile, Mn is an element for forming carbide,and may partially substitute for Fe atoms after entering into cementiteto increase the hardness of the carbide, thereby finally increasing thehardness of the steel rail. In the steel rail according to the presentinvention, when the content of Mn in the steel rail is less than 0.50%by weight, a strengthening effect is not satisfactory, and theperformances of the steel rail may be slightly improved only through thesolid solution effect. When the content of Mn is more than 1.20% byweight, the hardness of the carbide in the steel rail is too high sothat the steel rail may not obtain an ideal strength-toughness match,and more importantly, in a controlled cooling process duringmanufacturing the steel rail, carbon atoms in an austenite state may notbe sufficiently diffused at a relatively rapid cooling rate due to aneffect of Mn dragging solute atoms, thus a saturated or supersaturatedstate is formed, and abnormal structures such as bainite, martensitewhich are prohibited to occur in a pearlite-based steel rail, and thelike are easily generated. Therefore, the content of Mn is defined to bebetween 0.30% and 1.50% in the present invention, especially when 0.50wt %<Mn %<1.20 wt %, the effect is remarkable.

Al is prone to combine with oxygen in the steel to form Al₂O₃ or othercomplex oxides, which may remain in the steel if insufficientlyfloating, and which, as a heterogeneous phase, may damage continuity ofthe matrix when the steel rail is used. The inclusion forms a fatiguecrack source under a repeated stress, and further expanding of thefatigue crack source may increase a tendency of laterally brittlefracture of the steel rail. Therefore, the content of Al should notexceed 0.005% so as to improve the purity of the steel rail and toensure the safety.

RE (rare earth element) facilitates deformation of nonmetallicinclusions, while improving the purity of the steel. In addition, REalso decreases the damage of impurities such as S, As, etc. toproperties of steel products, and improves the fatigue property of arail steel. However, when the content of RE is more than 0.05%, it iseasy to promote generation of coarse inclusions, thereby seriouslydeteriorating properties of steel products. As for the steel rail for ahigh speed or quasi-high speed railway, it is highly important toimprove the steel purity and reduce the damage of nonmetallic inclusionsto the steel matrix. Therefore, in the present invention, the contentrange of RE added is defined to less than or equal to 0.05%, especiallywhen the content of RE is more than 0.010 wt % and less than 0.020 wt %,the effect is remarkable.

In the present invention, the total content of V, Cr and Ti is requiredto be not more than 0.20%. The reasons are as follows: themicrostructure and properties of the steel rail are directly determinedby the content of C as a main strengthening element of steel, and as thecontent of C decreases, the ratio of ferrite in the microstructuregradually increases and the ratio of pearlite decreases. Meanwhile, itis difficult for the ferrite as a soft phase in the steel to bearrepeated wear of the wheel, and even through a heat treatment, theincrement in strength of the ferrite matrix is also limited. Therefore,alloy elements such as V, Cr and/or Ti, etc. are required to be added tostrengthen the ferrite matrix so that the wear property may be improvedwhile improving toughness and plasticity of the rail. Hereinafter, thepurpose and range of adding the above three alloy elements will bedescribed in detail.

V in the steel has a very low solubility at the room temperature, andusually forms V(C, N) with C and N in the steel to refine grains and toimprove toughness and plasticity while strengthening the matrix, andthus is one of the strengthening elements usually used in the carbonsteel. In the steel rail according to the present invention, when thecontent of V is less than 0.15%, the above effects may be well achieved;when the content of V is further increased, the strength will be furtherimproved, while toughness, especially impact performance, issignificantly decreased, that is, the ability of the steel rail toresist impact is weakened, which is not suitable for high safetyrequired by the steel rail for high speed railway. When the content of Vis less than 0.01%, the strengthening effect is hardly to be exhibiteddue to a limited amount of the precipitated V. Thus, when V is addedalone, the content of V is defined in a range of 0.01% to 0.15%, andespecially when the content of V falls within a range of 0.02%≦V%≦0.08%, the effect is more remarkable.

Cr may form a continuous solid solution with Fe and form a variety ofcarbides with C, and is also one of primary strengthening elements inthe steel. In addition, Cr may allow the distribution of the carbides inthe steel to be uniform, and improve the wear property of the steel.Compared with V, Cr has a biggest advantage in economy. However, if thecontent of Cr is relatively high, welding performance may be adverselyaffected. In the present invention, the ratio of ferrite in the steelincreases due to the decrease in the content of C, and thussolid-solution strengthening elements are required to be added toimprove the strength of the ferrite so as to ensure the wear property ofthe rail in use. Meanwhile, since the high speed or quasi-high speedtrain has a light axle load, the wear is limited. Therefore, the contentof Cr is defined in a range of 0.02% to 0.20%, and especially when thecontent of Cr falls within a range of 0.10%≦Cr %≦0.15%, the effect ismore remarkable.

In the steel, Ti refines austenite grains during heating, rolling andcooling, and finally increases the toughness and plasticity of themetallurgical structure as well as rigidity. In the steel rail accordingto the present invention, when the content of Ti is more than 0.05%, TiCis excessively generated due to Ti being a strong element for formingcarbonitride, causing excessively high hardness of the steel rail, andon the other hand, excessive TiC may be concentrated to form coarsecarbides, not only reducing the toughness and plasticity, but alsomaking a contact surface of the steel rail be prone to crack andresulting in fracture under an impact load. In the steel rail accordingto the present invention, when the content of Ti is less than 0.01%, theamount of the formed carbonitride is limited, causing its effect to behardly exhibited. Therefore, in the present invention, the content of Tiis defined in a range of 0.01% to 0.05%.

The steel rail for the high speed or quasi-high speed railway has a lowstrength, required elements such as V, Cr, Ti and the like play limitedeffects of solid-solution strengthening and precipitation strengthening.Meanwhile, the toughness and plasticity has been significantly improveddue to the reduction in the carbon content in the present invention, andthe wear property of the steel rail may be improved only by the abovealloy elements. Accordingly, the total amount of V, Cr and Ti in thesteel rail is defined to be not more than 0.20% (0<V+Cr+Ti≦0.20%) in thepresent invention.

Hereinafter, a method for manufacturing a steel rail for high speed andquasi-high speed railways according to the present invention will bedescribed in detail.

According to the present invention, a method for manufacturing a steelrail for high speed and quasi-high speed railways according to thepresent invention includes the following steps.

(1) Smelting and Casting Molten Steel

First, a molten steel having the following composition is smelted byusing a converter, an electric furnace or an open-hearth furnace:0.40-0.64% of C, 0.10-1.00% of Si, 0.30-1.50% of Mn, less than or equalto 0.025% of P, less than or equal to 0.025% of S, less than or equal to0.005% of Al, more than 0 and less than or equal to 0.05% of a rareearth element (RE), more than 0 and less than or equal to 0.20% of atleast one of V, Cr, and Ti, and a remainder of Fe and inevitableimpurities. Then, after LF (Ladle Furnace) refining (i.e., secondaryrefining) and a vacuum treatment, the molten steel is cast to a billetor a slab, and the billet or the slab is cooled or directly transferredto a heating furnace to increase a temperature thereof.

(2) Rolling Steel Rail

The temperature of a continuously cast billet or slab is increased to acertain temperature (preferably 1200° C.-1300° C.) and kept for 0.5-2 h,and then the continuously cast billet or slab is fed into a rollingmachine to be rolled to a steel rail with a required cross-section.

(3) Controlled Cooling after Rolling

The steel rail is generally kept at a temperature of more than 800° C.after finishing rolling, and at this time, the steel rail may achievevarious performances by controlling a cooling rate of a rail headportion of the steel rail. For the steel rail still having surplus heatafter rolling, because of rolling characteristics of a rolling machine,the steel rail contacts a roll table at rail head side and rail basecorner of a side thereof, while only the rail head portion ispractically used. In the present invention, the controlled cooling isperformed by firstly making the steel rail stand upright on the rolltable, and transferring the steel rail to a heat treatment unit throughrotation of the roll table. Before this, nozzles of the heat treatmentunit for cooling a top side and both lateral sides of the rail headportion has started blowing cooling medium having appropriate pressureand flow rate, generally 2−15 kPa in an atmospheric environment. Whenthe steel rail goes through the nozzles sequentially arranged byrotation of the roll table, the rail head portion is uniformly cooled ata cooling rate of 1-4° C./s. When an infrared temperature detectingdevice located above the heat treatment unit detects a temperature ofthe top side of the rail head portion drops to 350-550° C., thecontrolled cooling is stopped, thereby completing the controlled coolingof the head portion of the steel rail.

In the present invention, a medium for accelerated cooling may be atleast one of compressed air, a mixture of water and air, and a mixtureof oil and air. Under the teaching of the present invention, thoseskilled in the art can determine the medium for accelerated cooling tobe used based on actual needs. Specifically, in the case of using thecompressed air and the mixture of water and air as the medium foraccelerated cooling, the ratio therebetween may be determined on thebasis of common selections.

(4) Air-Cooling

After the temperature of the head portion of the steel rail reaches atemperature range at which the accelerated cooling is finished, thesteel rail is placed in the air to be naturally cooled, and then istreated by subsequent processes.

In addition, an on-line heat treatment process is used in the above step(3). In the present invention, however, an off-line heat treatmentprocess may also be used. The off-line heat treatment is a process inwhich the steel rail is firstly air-cooled to a room temperature afterbeing rolled, and then heated by an induction heating device to atemperature in austenitic phase zone, typically 900-1100° C., andfinally the rail head portion is subjected to accelerated cooling. Inparticular, after a steel billet or slab is rolled into a steel rail bythe aforementioned steps, the steel rail is naturally cooled to atemperature lower than the austenitic phase zone, and then re-heated toa temperature falling in the austenitic phase zone or above 800° C.,followed by being subjected to the process of the step (3), therebyobtaining the product of the present invention as well. In the presentinvention, when a billet or slab is rolled into a steel rail and cooledto a temperature below the austenitic phase zone, the steel rail isheated to a temperature range of 800-1000□ at a rate of 1-20° C./s, andthen the process of step (3) is repeated, in which, uniformly cooling isperformed on the rail head portion at a cooling rate of 1-4° C./s and isstopped when the temperature of the rail head portion drops to 350-550°C., and subsequently the steel rail is naturally cooled to the roomtemperature in the air. Here, it should be noted that when the steelrail naturally cooled is re-heated to a temperature in the austeniticphase zone, various heating rates may be applied based on factors suchas specific equipment conditions, etc., for example, the steel rail canbe either slowly heated to a temperature in the austenitic phase zone ata rate of 1V/s, or rapidly heated to a temperature in the austeniticphase zone at a rate of 20° C./s.

The method of manufacturing a steel rail according to the presentinvention is substantially the same as that of the prior art, except forthe step of controlled cooling after rolling, and thus detaileddescription of identical contents will be omitted. In the presentinvention, after the finishing rolling, the rail head portion isuniformly cooled at a cooling rate of 1-4° C./s, and when thetemperature of the rail head portion drops to 350-550° C., the coolingis stopped. Performances of a final product is determined by theselection on the cooling processes, and thus in the present invention,when the steel rail containing the above components is cooled at a rateof less than 1 □/s, a strength of the steel rail equivalent to that ofan existing steel rail for a high speed or quasi-high speed railwaycannot be achieved by refining ferrite and pearlite grains in themicrostructure, and an insufficient ferrite matrix strength may causethe steel rail in use to hardly bear vertical loads of a train, so thata top side of a rail head portion has a size deviation due to plasticflow, while generating excessive wear, which not only reduces a servicelife of the steel rail, but also endangers running safety. On the otherhand, when the cooling rate is more than 4° C./s, the diffusion rate ofthe carbides in the steel reduces to increase a possibility ofgeneration of bainite and martensite structures which are expresslyprohibited to occur in a pearlite-based steel rail. Moreover, if thecooling rate is too high, the strength of the steel rail will besignificantly increased, and although energy required for crackinitiation and propagation may be increased at the same time, crackswhich have been generated can not be removed by wear between the wheeland the rail, adversely affecting the running safety.

In the present invention, the temperature at which the acceleratedcooling is terminated is 350-550° C. for the reasons as follow. Thesteel rail containing the above components is accelerated cooled fromthe austenite phase zone, and phase transition has been completed at arail surface to a depth of at least 15 mm below the surface at about550° C.; at this time, heat existing inside the rail head portion willbe transferred outwards, and if the accelerated cooling is terminated,the temperature of the surface of the rail may rise due to thermalconduction such that the refined microstructure which has formed isroughened, not facilitating transition of the internal microstructure ofthe rail head portion at a relatively great degree of supercooling, andthus the effect of heat treatment can not be fully achieved. If thetemperature at which the accelerated cooling is terminated is lower than350° C., the steel rail has entered into a bainite transformation zone,which is not conducive to obtain stable pearlite and ferritemicrostructures, thereby increasing a tendency of generating abnormalmicrostructures.

In the present invention, the accelerated cooling is performed only on arail head portion, while a rail waist and a rail base are subjected tonatural air-cooling to reach a room temperature for reasons as follow.The rail waist of the steel rail, as a connector between the rail headportion and the rail base, indirectly receives a load from a train andneeds to have a certain stiffness, while it also receives a normal forcegenerated by steering the train. The rail base applies a force directlyto railway sleepers to determine a running trajectory of the train, andfinally transfers the load to a track bed. As for the high speed andquasi-high speed railways, a train has an axle load of 11 t-14 t lowerthan an axle load of 25 t-40 t of a train traveling on a mixed passengerand freight railway or a heavy haul railway, and has a large line curveradius of greater than typically 1000 m, and the rail waist and the railbase can bear limited vertical and normal forces. In addition, theaccelerated cooling has a limited effect on toughness and plasticityindices and has no significant effect on the safety of the steel rail inuse as compared with air-cooling.

The steel rail obtained by using the method of manufacturing a steelrail according to the present invention may have a mixed microstructureof fine pearlite and fine ferrite (15%-50%) in the rail head, have astrength reaching an equivalent level of strength of an existing steelrail for a high speed or quasi-high speed railway while significantlyimproving toughness and plasticity and yield strength thereof, improvethe ability to resist impact loads while increasing the energy requiredfor crack initiation and propagation of a surface layer of the steelrail, and ultimately improve the rolling contact fatigue properties toprotect the transporting safety of the railway. Meanwhile, the methodaccording to the present invention requires no modification in theexisting equipments during the manufacturing processes, and thus themanufacturing processes are simple, convenient and flexible.

Hereinafter, the present invention will be described in more detail inconjunction with examples. These examples are for illustrative purposesonly and are not intended to limit the scope of the present invention.

Example 1

To obtain a steel rail having a composition as listed in Table 2 below,smelting by a converter, LF refining, vacuum degassing, continuouscasting for billet, heating by a billet heating furnace, and railrolling were sequentially performed, wherein the steel rail was rolledat a finishing rolling temperature of 903° C. and then was placed for 40seconds; after that, when a temperature of a top surface of a rail headportion decreased to 800° C., compressed air began to be blown so as touniformly cool the rail head portion at a cooling rate of 3.1° C./s; andwhen the temperature of the top surface of the rail head portion reached520° C., and temperatures of a rail waist and a rail base wererespectively greater than 600° C. after blowing, the steel rail wasplaced in the air to be naturally cooled to a room temperature, therebyobtaining Sample 1.

Example 2

Except for steps of controlled cooling after rolling, a steel rail wasmanufactured by using the same method as that in Example 1.Specifically, in this example, the steel rail was rolled at a finishingrolling temperature of 910° C. and then was placed for 45 seconds; afterthat, when a temperature of a top surface of a rail head portiondecreased to 780° C., compressed air and a mixture of oil and air beganto be blown so as to uniformly cool the rail head portion at a coolingrate of 2.9° C./s; and when the temperature of the top surface of therail head portion reached 514° C., and temperatures of a rail waist anda rail base were respectively greater than 600° C. after blowing, thesteel rail was placed in the air to be naturally cooled to a roomtemperature, thereby obtaining Sample 2.

Example 3

Except for steps of controlled cooling after rolling, a steel rail wasmanufactured by using the same method as that in Example 1.Specifically, in this example, the steel rail was rolled at a finishingrolling temperature of 900° C. and then was placed for 42 seconds; afterthat, when a temperature of a top surface of a rail head portiondecreased to 770° C., a mixture of oil and air began to be blown so asto uniformly cool the rail head portion at a cooling rate of 2.7° C./s;and when the temperature of the top surface of the rail head portionreached to 530° C., and temperatures of a rail waist and a rail basewere respectively greater than 600° C. after blowing, the steel rail wasplaced in the air to be naturally cooled to a room temperature, therebyobtaining Sample 3.

Example 4

Except for steps of controlled cooling after rolling, a steel rail wasmanufactured by using the same method as that in Example 1.Specifically, in this example, the steel rail was rolled at a finishingrolling temperature of 890° C. and then was placed for 35 seconds; afterthat, when a temperature of a top surface of a rail head portiondecreased to 790° C., a mixture of water and air and a mixture of oiland gas began to be blown so as to uniformly cool the rail head portionat a cooling rate of 3.0° C./s; and when the temperature of the topsurface of the rail head portion reached to 495, and temperatures of arail waist and a rail base were respectively greater than 550° C. afterblowing, the steel rail was placed in the air to be naturally cooled toa room temperature, thereby obtaining Sample 4.

Example 5

Except for steps of controlled cooling after rolling, a steel rail wasmanufactured by using the same method as that in Example 1.Specifically, in this example, the steel rail was rolled at a finishingrolling temperature of 915° C. and then was placed for 50 seconds; afterthat, when a temperature of a top surface of a rail head portiondecreased to 780° C., compressed air began to be blown so as touniformly cool the rail head portion at a cooling rate of 2.8° C./s; andwhen the temperature of the top surface of the rail head portion reachedto 528° C., and temperatures of a rail waist and a rail base wererespectively greater than 600° C. after blowing, the steel rail wasplaced in the air to be naturally cooled to a room temperature, therebyobtaining Sample 5.

Example 6

Except for steps of controlled cooling after rolling, a steel rail wasmanufactured by using the same method as that in Example 1.Specifically, in this example, the steel rail was rolled at a finishingrolling temperature of 922° C. and then was placed for 53 seconds; afterthat, when a temperature of a top surface of a rail head portiondecreased to 795° C., compressed air began to be blown so as touniformly cool the rail head portion at a cooling rate of 2.1° C./s; andwhen the temperature of the top surface of the rail head portion reachedto 519° C., and temperatures of a rail waist and a rail base wererespectively greater than 600° C. after blowing, the steel rail wasplaced in the air to be naturally cooled to a room temperature, therebyobtaining Sample 6.

Example 7

Except for steps of controlled cooling after rolling, a steel rail wasmanufactured by using the same method as that in Example 1.Specifically, in this example, the steel rail was rolled at a finishingrolling temperature of 918° C. and then was placed for 49 seconds; afterthat, when a temperature of a top surface of a rail head portiondecreased to 800° C., compressed air began to be blown so as touniformly cool the rail head portion at a cooling rate of 2.2° C./s; andwhen the temperature of the top surface of the rail head portion reachedto 531° C., and temperatures of a rail waist and a rail base wererespectively greater than 600° C. after blowing, the steel rail wasplaced in the air to be naturally cooled to a room temperature, therebyobtaining Sample 7.

Example 8

Except for steps of controlled cooling after rolling, a steel rail wasmanufactured by using the same method as that in Example 1.Specifically, in this example, the steel rail was rolled at a finishingrolling temperature of 907° C. and then is placed for 48 seconds; afterthat, when a temperature of a top surface of a rail head portiondecreased to 785° C., compressed air and a mixture of water and airbegan to be blown so as to uniformly cool the rail head portion at acooling rate of 2.3° C./s; and when the temperature of the top surfaceof the rail head portion reached to 526° C., and temperatures of a railwaist and a rail base were respectively greater than 600° C. afterblowing, the steel rail was placed in the air to be naturally cooled toa room temperature, thereby obtaining Sample 8.

Example 9

Except for steps of controlled cooling after rolling, a steel rail wasmanufactured by using the same method as that in Example 1.Specifically, in this example, the steel rail was rolled at a finishingrolling temperature of 895° C., was firstly air-cooled to a roomtemperature, and then a rail head portion was re-heated to 900° C. byusing a line-frequency induction heating device at a rate of 5° C./s;after that, when the rail head portion was naturally air-cooled to 760°C., a mixture of water and air and compressed air were blown so as touniformly cool the rail head portion at a cooling rate of 2.2° C./s; andwhen the temperature of the top surface of the rail head portion reached510° C., and temperatures of a rail waist and a rail base wererespectively greater than 600° C. after blowing, the steel rail wasplaced in the air to be naturally cooled to a room temperature, therebyobtaining Sample 9.

Comparative Example 1

Except for steps of controlled cooling after rolling, a steel rail wasmanufactured by using the same method as that in Example 1. After beingrolled into a desired section, the steel rail was directly placed in airto be cooled to a room temperature, thereby obtaining an existing steelrail for a high speed or quasi-high speed railway of Comparative Example1.

TABLE 2 Chemical compositions of the steel rails according to thepresent invention and Comparative Example 1 Chemical compositions (%, byweight) No. C Si Mn P S Al RE V Cr Ti V + Cr + Ti Examples of the 1 0.560.16 1.08 0.021 0.012 0.002 0.004 0.010 0.03 0.005 0.045 presentinvention 2 0.45 0.20 1.17 0.019 0.009 0.003 0.015 0.020 0.16 0.0060.186 3 0.48 0.30 0.87 0.016 0.007 0.003 0.005 0.030 0.10 0.008 0.138 40.60 0.15 0.85 0.017 0.008 0.002 0.012 0.010 0.03 0.008 0.048 5 0.520.49 0.53 0.020 0.010 0.003 0.009 0.030 0.11 0.010 0.150 6 0.64 0.300.78 0.014 0.005 0.004 0.011 0.002 0.04 0.004 0.046 7 0.62 0.25 0.750.013 0.009 0.005 0.010 0.005 0.02 0.015 0.040 8 0.42 0.50 1.19 0.0150.006 0.003 0.013 0.035 0.02 0.006 0.061 9 0.48 0.30 0.87 0.016 0.0070.002 0.010 0.040 0.03 0.011 0.081 Comparative 1 0.71 0.25 1.20 0.0180.010 0.004 — 0.010 0.03 0.004 0.044 Example

Experimental Example 1

Mechanical properties of the steel rails according to the presentinvention and the prior art are shown in Table 3 below.

TABLE 3 Mechanical properties of the steel rails according to thepresent invention and Comparative Example 1 Hardness Elonga- of topTensile Yield tion surface strength strength after of steelMetallurgical (R_(m), (R_(el), fracture rail No. structure MPa) MPa) (A,%) (HB) Examples 1 Pearlite + 950 580 20.0 265 of the 24% ferritepresent 2 Pearlite + 930 575 21.5 255 invention 37% ferrite 3 Pearlite +950 595 19.0 263 32% ferrite 4 Pearlite + 980 605 17.0 279 19% ferrite 5Pearlite + 960 590 18.0 270 28% ferrite 6 Pearlite + 990 600 16.5 28016% ferrite 7 Pearlite + 970 590 17.5 276 18% ferrite 8 Pearlite + 930580 22.0 257 38% ferrite 9 Pearlite + 980 610 18.0 276 30% ferriteCompar- 1 Pearlite + 950 550 12.0 275 ative ferrite (<5%) Example

It can be seen from Table 3 above that the steel rails of Examples 1 and3 according to the present invention have strengths at the same levelwith the steel rail of Comparative Example 1, but have elongationsincreased by about 50% than the steel rail of Comparative Example 1. Thesteel rails of Examples 2 and 8 according to the present invention havetensile strengths (R_(m)) slightly lower than the steel rail ofComparative Example 1, but have yield strengths (R_(el)) higher than thesteel rail of Comparative Example 1, this will effectively preventsurface fatigue cracks from being generated in the steel rails in useunder the same conditions; meanwhile, the steel rails of Examples 2 and8 may satisfy wear requirements since the practical wear of a steel railfor a high speed railway is small due to a low contact stress betweenthe rail and the wheels. Furthermore, the steel rail of Example 2according to the present invention has an elongation after fractureincreased by about 75% than that of the steel rail of ComparativeExample 1, thereby improving the safety in use. Compared withComparative Example 1, the steel rails of Example 4, Example 6, Example7 and Example 8 in the present invention have improved strengths andhardnesses, while having plasticities significantly improved, so thatthe overall performances are improved. As for Example 9 using secondaryheating, its performances may also meet the requirements of steel railsfor a high speed or quasi-high speed railway because ferrite grains arerefined.

FIG. 2 is a metallograph of a rail head structure of the steel rail ofExample 1 according to the present invention. FIG. 3 is a metallographof a steel rail head structure of the steel rail according toComparative Example 1. It can be seen from FIGS. 2 and 3 that the steelrail manufactured by the method according to the present invention has amicrostructure in which pearlite and ferrite are mixed and arrangeduniformly, as compared with the steel rail according to ComparativeExample 1. Thus, in the steel rail of the present invention, the wearproperty of the steel rail may be improved by cementite in pearlite, andthe toughness and fatigue properties may be improved at the same time bystrengthened ferrite. Therefore, as for steel rails used for high speedand quasi-high speed railways, the steel rail according to the presentinvention has relatively better resistance to wear and resistance tocontact fatigue than the steel rail according to the prior art.

Experimental Example 2

Impact energies (Ak_(u)) at different temperatures of the steel railsaccording to the present invention and the prior art are shown in Table4 below.

TABLE 4 Impact energies at different temperatures of the steel railsaccording to the present invention and Comparative Example 1 Impactenergies at different temperatures (Ak_(u)/J) No. 20° C. 0° C. Examplesof 1 30 20 the present 2 39 28 invention 3 32 23 4 25 19 5 34 21 6 28 207 28 21 8 40 31 9 32 20 Comparative 1 20 13 Example

It can be seen from Table 4 above that, as compared with the steel railmanufactured according to the prior art, the steel rails manufactured bythe method according to the present invention have significantlyimproved impact toughness at normal and low temperatures, andespecially, the toughnesses of the steel rails in Example 2 and Example8 have been increased to be nearly doubled due to the use of low carboncontent and a micro-alloying process. As for the steel rails accordingto Examples 4 and 6 having relatively high carbon contents withoutalloying, the impact toughnesses are also improved by 25%. Thus, it canbe seen that the reduction in the carbon content and the controlledcooling after rolling are advantageous to improve the toughness of therail steel. Therefore, the steel rail manufactured by the method of thepresent invention can provide more effective protection for use safetyof trains traveling on high speed railways in a cold area regardless ofimpact between the rail and the wheel resulting from irregular railwayconditions or other reasons.

Experimental Example 3

Wear properties of the steel rails according to the present inventionand the prior art are shown in Table 5 below.

The steel rails according to the present invention were ground againstthe steel rail of the prior art as a comparative sample by means ofrolling-sliding wear so that the wear properties of the steel rails arecompared at the same conditions. The specific experimental conditionsand parameters are as follow:

Type of a test device: Type MM-200;

Sizes of samples: a thickness of 10 mm, an inner diameter of 10 mm, andan outer diameter of 36 mm;

Testing load: 980N;

Sliding difference: 10%;

Testing environment: at a normal temperature and air cooling;

Rotating speed: 200 r/min;

Total rotating numbers of grinding: 200,000; and

Numbers of testing objects: three pairs (their arithmetic mean valueswere calculated as results).

The results for wear testing are shown in Table 5, and a schematic viewshowing the wearing is shown in FIG. 1.

TABLE 5 Wear properties of the steel rails according to some examples ofthe present invention and Comparative Example 1 Loss of weight afterwearing (g) Serial No. No. 1 2 3 1 Example 5 1.3198 1.3509 1.2956Comparative Example 1 1.3271 1.3596 1.2988 Ratio of lost weight 99.45%99.36% 99.76% 2 Example 6 1.4140 1.4374 1.4193 Comparative Example 11.4525 1.4714 1.4635 Ratio of lost weight 97.35% 97.69% 96.98% 3 Example8 1.2813 1.2855 1.2405 Comparative Example 1 1.2409 1.2286 1.1985 Ratioof lost weight 103.26% 104.63% 103.50%

It can be seen from Table 5 above that the wear property of the steelrail of Example 8 in the present invention is slightly inferior to thatof Comparative Example 1. Since a high speed train has a relativelylighter axle load and a steel rail for the high speed train has arelatively lower wear rate, a relatively lower wear property facilitatesto remove fatigue cracks generated at a surface of a rail head portionof the steel rail by wearing, and thus greatly helps to improve therolling contact fatigue property. Wear properties of the steel railsaccording to Examples 5 and 6 are equivalent to the wear property of thesteel rail of Comparative Example 1, and thus the steel rails accordingto Examples 5 and 6 are also suitable for high speed or quasi-high speedrailway applications.

Experimental Example 4

Fatigue crack propagating rates of the steel rails according to thepresent invention and the prior art are shown in Table 6 below. A devicefor testing crack propagating rate, ISTRON 8801, was used to study arule of a rate at which a length or depth of cracks propagates in adirection vertical to a stress direction. The slower the crackpropagating rates are, the more beneficial to prevent the cracks frompropagating under the same conditions.

TABLE 6 Fatigue crack propagating rates of the steel rails according tothe present invention and Comparative Example 1 da/dN (M/GC) at da/dN(M/GC) at ΔK = 10 MPam^(1/2) ΔK = 13.5 MPam^(1/2) Average Average No.Range value Range value Examples of the 5 2.77~3.68 3.32 16.20~19.8517.90 present invention 6 2.89~3.87 3.44 17.66~20.56 18.25 8 2.75~3.353.05 15.85~19.05 17.65 9 3.05~3.94 3.42 18.55~21.22 19.45 Comparative 14.56~5.75 5.08 22.88~24.56 23.60 Example

It can be seen from Table 6 above that the steel rails manufactured bythe method according to the present invention have a crack propagatingrate lower than that of the steel rail in Comparative Example 1, andthus the present invention may help to prevent cracks from propagatingunder the same conditions.

Experimental Example 5

Fracture toughnesses (K_(IC)) at a low temperature (−20° C.) and anormal temperature (20° C.) of the steel rails according to the presentinvention and the prior art are shown in Table 7 below. A device fortesting fracture toughness, ISTRON 8801, was used to measure thefracture toughnesses. The fracture toughness K_(IC) is a mechanicalproperty index exhibiting an ability of a material to resist crackpropagation. The higher the value of K_(IC) is, the stronger the abilityof the steel rail to resist crack propagation and the safer the trainruns.

TABLE 7 Fracture toughnesses of the steel rails according to the presentinvention and Comparative Example 1 K_(IC) at 20° C. K_(IC) at −20° C.(MPam^(1/2)) (MPam^(1/2)) Average Average No. Range value Range valueExamples of the 5 42~47 44.2 40~45 42.3 present invention 6 40~44 41.239~42 40.5 8 44~50 47.6 42~47 44.4 9 42~45 43.3 41~45 42.0 Comparative 134~38 36.8 32~36 34.9 Example

It can be seen from Table 7 above that the fracture toughnesses of thesteel rails manufactured according to the method of the presentinvention are higher than that of the steel rail of Comparative Example1 under the same conditions, at both the normal temperature and the lowtemperature. By comparison, it can be found that the fracture toughnessis significantly improved as the carbon content in the steel reduces.Therefore, the reduction in the carbon content of the steel rail helpsto obtain higher fracture toughness.

Experimental Example 6

Axial fatigue performances of the steel rails according to the presentinvention and the steel rail of Comparative Example 1 are shown in Table8 below. Axial fatigue performances of the steel rails were measured byusing a method of increasing and decreasing a stress amplitude by a PQ-6bending fatigue testing machine under a testing condition that eachgroup of samples has a fatigue lifetime greater than 5×10⁶ when a totalstrain amplitude is 1350με.

TABLE 8 Axial fatigue limits of the steel rails according to the presentinvention and Comparative Example 1 No. Axial fatigue limits (MPa)Examples of the 5 352.8 present invention 6 347.6 8 353.5 9 340.5Comparative 1 332.5 Example

It can be seen from Table 8 above that both the steel rails manufacturedaccording to the method of the present invention and the steel railmanufactured according to the prior art meet standard requirements, andthe fatigue limits of the steel rails according to the present inventionare higher than the fatigue limit of the steel rail manufacturedaccording to the prior art.

In the existing steel rail for high speed and quasi-high speed railways,the rail head portion has a microstructure of a great amount of pearliteand less than 5% of ferrite, whereas according to the steel rail forhigh speed and quasi-high speed railways according to the presentinvention, the rail head portion has a uniformly mixed microstructure ofpearlite and 15% to 50% of ferrite at the room temperature by reducingthe content of C in the steel rail in conjunction with the controlledcooling after rolling. The steel rail for high speed railways includesferrite having a ratio increased to 15% to 50% in the microstructure.This is advantageous in that: (1) the existing steel rail for high speedrailways has a microstructure containing a dominant component ofpearlite and less than 5% of a ferrite structure, and it has been foundthat wear between the high speed trains and rails barely occurs during acertain period of running, resulting in that it is difficult for thepearlite structure with significantly good wear properties to play itsrole, and on the contrary, microcracks generated at a rail head surfacecontacting the wheels will be hardly removed because of no wear, but mayexpand toward the inside of the steel rail under repeated action fromthe wheels, and finally form contact fatigue damages such as cracks,drops, etc., which may cause a risk of broken rail. When the ratio ofthe ferritic structure increases, since ferrite belongs to a soft phasein the steel and has a wear property far inferior to pearlite, the steelrail may have a certain wear generated in use so as to ensure the cracksat the surface of the steel rail to be worn away timely. However, if acertain ratio of ferrite is obtained by simply decreasing the content ofC in the steel, the service life of the steel rail may also be adverselyaffected due to excessive wear. Thus, the expected effect can beachieved only by strengthening the ferrite matrix, and in order toimprove the strength of the matrix, there are three ways, i.e., solidsolution strengthening of alloy elements, precipitation strengthening,and grain refining strengthening by a heat treatment. If a heattreatment process is performed alone, a strengthening effect fromcementite may be enhanced while the strength of the ferrite matrix isincreased, which may cause an excessively high strength. Thus, somemicro-alloying elements are added to mostly strengthen the ferritematrix, while slightly improving toughness and plasticity. In addition,if the ratio of ferrite exceeds 50%, the ratio of pearlite will bedecreased, which cannot ensure a certain degree of the wear property,also causing the steel rail incapable of being applied to high speedrailways. (2) The increase in the ratio of ferrite in the steel railmeans a significant enhancement of the toughness and plasticity, and arelatively higher elongation as well as impact toughness will greatlyreduce a possibility of broken rail under the same impact load, which isdefinitely beneficial to ensure the running safety.

In summary, by comparing the metallurgical microstructures, commonmechanical properties and special mechanical properties of the steelrail according to the present invention under various conditions withthose of the existing steel rail for high speed railways, it can be seenthat, in the present invention, by reducing the content of C element inthe steel rail in conjunction with the controlled cooling after rolling,the levels of strength and hardness of the existing steel rail for highspeed railways are maintained, meanwhile, both the toughness andplasticity and the yield strength of the steel rail are remarkablyimproved, that is, the energy value required for initiating andexpanding microcracks formed at the surface of the steel rail due tofatigue can be increased, and thus under the same conditions, therolling contact fatigue property of the to steel rail can be improved,thereby finally improving the service lifetime and the transportationsafety of the steel rail.

The present invention is not limited to the above embodiments, andvarious variation and modifications can be made therein withoutdeparting from the scope of the present invention.

1. A steel rail for high speed and quasi-high speed railways, comprising0.40-0.64% by weight of C, 0.10-1.00% by weight of Si, 0.30-1.50% byweight of Mn, less than or equal to 0.025% by weight of P, less than orequal to 0.025% by weight of S, less than or equal to 0.005% by weightof Al, more than 0 and less than or equal to 0.05% by weight of a rareearth element, more than 0 and less than or equal to 0.20% by weight ofat least one of V, Cr, and Ti, and a remainder of Fe and inevitableimpurities, wherein a head portion of the steel rail has a uniformlymixed microstructure of pearlite and 15-50% of ferrite at a roomtemperature.
 2. The steel rail of claim 1, comprising 0.45-0.60% byweight of C, 0.15-0.50% by weight of Si, 0.50-1.20% by weight of Mn,less than or equal to 0.025% by weight of P, less than or equal to0.025% by weight of S, less than or equal to 0.005% by weight of Al,more than 0 and less than or equal to 0.05% by weight of a rare earthelement, more than 0 and less than or equal to 0.20% by weight of atleast one of V, Cr, and Ti, and a remainder of Fe and inevitableimpurities.
 3. The steel rail of claim 1, comprising at least one of0.01-0.15% of V, 0.02-0.20% of Cr, and 0.01-0.05% of Ti.
 4. The steelrail of claim 3, comprising at least one of 0.02-0.08% of V, 0.10-0.15%of Cr, and 0.01-0.05% of Ti.
 5. The steel rail of claim 1, wherein thehead portion of the steel rail has a uniformly mixed microstructure ofpearlite and 15-30% of ferrite at the room temperature.
 6. A method ofmanufacturing the steel rail of claim 1, comprising smelting and castingmolten steel, rolling steel rail, controlled cooling after rolling, andair-cooling, wherein the controlled cooling after rolling comprisesmaking the steel rail stand upright on a roll table, transferring thesteel rail to a heat treatment unit through rotation of the roll table,and blowing cooling medium onto the steel rail by the heat treatmentunit to uniformly cool the head portion of the steel rail at a coolingrate of 1-4° C./s until a temperature of a top side of the head portiondecreases to 350-550° C.
 7. The method of claim 6, further comprisingafter finishing rolling during the rolling steel rail, cooling the steelrail to a temperature lower than an austenitic phase zone, and thenheating the steel rail to a temperature in the austenitic phase zone ata rate of 1-20° C./s, followed by the controlled cooling after rolling.8. The method of claim 6, wherein the cooling medium is at least one ofcompressed air, a mixture of water and air, and a mixture of oil andair.
 9. The method of claim 6, wherein the head portion of the steelrail finally obtained has a uniformly mixed microstructure of pearliteand 15-30% of ferrite at a room temperature.
 10. The method of claim 6,wherein the smelting and casting molten steel comprises smelting themolten steel by using a converter, an electric furnace or an open-hearthfurnace, performing a vacuum treatment on the molten steel, casting themolten steel to a billet or a slab, and cooling the billet or the slabor directly transferring the billet or the slab to a heating furnace toincrease a temperature thereof.
 11. The method of claim 6, wherein therolling steel rail comprises feeding a billet or a continuous cast slabwhich has been heated to a certain temperature and kept for a certainperiod of time into a rolling machine to roll the billet or thecontinuous cast slab to a steel rail having a required cross-section.12. The method of claim 11, wherein during the rolling steel rail, thetemperature of the billet or the continuous cast slab is increased to1200-1300° C., and kept for 0.5-2 h.
 13. The method of claim 6, furthercomprising after the controlled cooling after rolling, placing thecooled steel rail in the air to be naturally cooled to a roomtemperature.
 14. The steel rail of claim 2, comprising at least one of0.01-0.15% of V, 0.02-0.20% of Cr, and 0.01-0.05% of Ti.